Devices are known for separating the tire beads from the rims on which they sit.
As is known, a tire bead is the part of the tire sidewall that extends from the circumference of the inner edge and the outer circumference connecting the sidewall to the tread and that, with the tire inflated, is designed to permanently lie in secured fashion on the peripheral edge of a wheel rim on which the tire sits.
Typically, after intensive use of the tires, due to the constant thrust exerted against the rim edges, beads tend to maintain their adhesion strength thereupon.
For this reason, when a tire changer has to demount a tire from its rim, he/she will first deflate it and then repeatedly press its beads toward the central tire fitting groove of the rim, using a pneumatically-operated blade-shaped press tool, with which tire-changing machines are equipped on one sidewall of their base, thereby loosening the tire from the rim.
Then, he/she will lay and lock the wheel with the loosened tire on the working platform provided in every tire-changing machine, move the operating arm of the tire-changing machine, having a bead-breaking tool, toward the tire bead, and hold a specially shaped lever to manually lift a small portion of the upper bead from the rim edge to pass over it, and fit the lifted portion onto the bead breaking tool.
Finally, he/she rotates the platform and the tire bead will progressively pass over the entire edge of the rim, thereby allowing tire demounting.
In the prior art, automatically operating bead breaking devices are also known, which allow operators to avoid the use of the lever.
A device of this type is disclosed in EP 1 593 533.
This patent provides a vertical operating arm which carries a pair of vertical and parallel plates, with a hook mounted therebetween, which is connected to the plates by means of two connecting rods, thereby forming an articulated quadrilateral in which the hook forms an outer side.
The hook has a roto-translational motion and is driven by two gears mounted between one of the connecting rods and the two parallel plates, said gears being rotatably integral with the connecting rod and a pivot transversely supported between the two vertical and parallel plates.
The two gears have respective teeth, which are mutually engaged for rotary motion transfer.
The pivot also has the end of an additional coupling rod keyed thereon, said connecting rod having its opposite end hinged to the lower end of the shaft of an actuator cylinder, which is mounted along the operating arm and integral therewith.
When the cylinder is actuated by the tire changer, the additional coupling rod rotates and causes integral rotation of the pivot and the gear rotatably fixed thereto.
The rotation of this gear forces the second gear into rotation, and the latter rotates the two connecting rods that form the articulated quadrilateral.
Therefore, the hook is moved toward or away from a predetermined introduction point between a tire bead and the edge of a rim on which it sits.
The compound roto-translational motion allows the hook to perform introduction or extraction movements between the bead and the rim edge along a path that passes over the rim of the edge, without impacting it.
Therefore, when the hook is moved back for extraction, it catches the tire bead and lifts it over a portion beyond the rim edge, thereby actually automatically carrying out the step that a tire changer carries out when he/she handles the bead breaking lever, to place the lifted bead portion on a bead breaking tool with which the tire-changing machine is equipped.
This prior art suffers from certain drawbacks.
A first drawback consists in that, in tire changing machines in which a lever must be manually operated to lift the tire bead segment to be demounted from the rim edge, tire changers are required to use their physical strength and when multiple tire demounting steps must be carried out in one working day, the effort progressively accumulates and becomes significant at the end of the working day.
This drawback is even more serious when the tires to be demounted are of special type, i.e. tires having lower sidewalls than standard tires.
Bead lifting by the lever becomes even more difficult and wearing due to the structural rigidity of the carcass of this type of tires, which must be able to limit its elastic lateral bending to limit the cornering slip angle of the vehicle to which they are mounted.
A further drawback of prior art automatic bead breaking devices, i.e. those that do not require manual use of the lever, particularly as described above, consists in that they have a very complicated structure, composed of a considerable number of parts connected together by hinges, which also include, as mentioned above, gears with mutually engaged teeth.
This involves both considerable costs for making the individual parts and assembling them to form the bead breaking device, and, due to wear, progressive increase of the clearances between the parts and the great number of hinges that connect them together, as well as the teeth of the gears that transfer the action of the actuator cylinder to the hook to convert it into roto-translational movements.
This increase of the clearances is directly proportional to the number of parts that are used to form the bead breaking device and worsens inaccuracy of the action of the hook that is designed first to fit between the bead and the rim edge, and then to lift the bead portion.
In certain cases, this inaccuracy may cause a considerable deviation of the operating roto-translational paths originally designed for the hook, thereby possibly causing it to impact or slip against the rim edge and damage it, during both introduction and extraction.
Furthermore, such deviations of the optimal paths change the angle of introduction of the hook between the tire bead and the rim edge.
Such angle changes may increase the bending resistance of the bead, upon pressure of the hook, to as it is moved toward the tire fitting groove, and conversely cause an unsteady bead hooking action during the extraction lifting step.